A young child may be puzzled when asked to distinguish between the directions of right and left. But that same child may have no difficulty in determining the directions of up and down or back and front. Scientists propose that this occurs because, although we experience three dimensions, only two had a strong influence on our evolution: the vertical dimension as defined by gravity and, in mobile species, the front/back dimension as defined by the positioning of sensory and feeding mechanisms. These influence our perception of vertical versus horizontal, far versus close, and the search for dangers from above (such as an eagle) or below (such as a snake). However, the left-right axis is not as relevant in nature. A bear is equally dangerous from its left or the right side, but not if it is upside down. In fact, when observing a scene containing plants, animals, and man-made objects such as cars or street signs, we can only tell when left and right have been inverted if we observe those artificial items.